(a) Field of the Invention
The present invention relates to an electric car. More specifically, the present invention relates to a device and method for charging a battery pack of an electric car.
(b) Description of the Related Art
Different from an internal combustion engine driven car, an electric car moves using the limited energy of a battery comprised of modules. When the energy is exhausted, the battery must be charged with external power.
FIG. 1 is a voltage waveform graph of a battery charging method according to prior art provided by the Hawker Company. As shown, the battery is charged in the three stages of constant power (CP), a first constant voltage (CV1), and a second constant voltage (CV2).
The switching of each of these charging stages is as follows.
When a charging process is performed using the constant power and the voltage of the battery pack reaches a value equivalent to 14.7 volts (V).times. number of modules, the first constant power stage is switched to the second stage. It takes about two hours for the switch to occur from the first stage to the second stage.
In the second stage, the battery is charged at a constant voltage of 14.7 volts. The completion of the second stage takes about 2.5 times the duration of the first stage, that is, about 5 hours.
In the third stage, the battery is charged at a constant voltage of 15.8 volts. The completion of the third stage takes about 0.5 times the duration of the first stage, that is, about one hour, and cannot exceed one hour.
However, as shown in FIG. 1, it is impossible to monitor the battery modules because of severe unbalances associated with the second and third stages which are caused by the different gassing characteristics of the battery modules. Also, since control operations in the second and third stages are performed according to elapsed time, diffusion can occur in those cases where any of the modules fail to charge.
In the event that a state of charge (SOC) of a lead acid battery is over 80%, charging voltage characteristics vary depend on input current because of different individual module gassing characteristics. This phenomenon leads to problems in battery charge control and residual capacity computation since a battery management system (BMS) is influenced by battery imbalance and failure phenomena.